If you feel you need that time to yourself then
absolutely go for it. I'm a single parent but to children younger and less self-reliant than yours so my chances for "time off" are more limited. But when I do get that chance, even if it's just a day or even just an afternoon, I grab it with both hands as I really do benefit from a bit of me-time. I understand the guilty feelings, but that's tempered because my kids seem to benefit from some time away from me, too - I like to think that's just because when they see how other families work, they appreciate how fantastic a dad I am. I hope.
You're not just a mother and an employee, you're a person in your own right. You are allowed to do things that are good for you, too.
Mr B.